Cryptographic Hash Function

Hashing as a tool to associate one set or bulk of data with an identifier has many different forms of application in the real-world. Cyptographic Hashing is used for data/user verification and authentication. A strong cryptographic hash function has the property of being very difficult to reverse the result of the hash and hence reproduce the original piece of data. Cryptographic hash functions are used to hash user's passwords and have the hash of the passwords stored on a system rather than having the password itself stored. Cryptographic hash functions are also seen as irreversible compression functions, being able to represent large quantities of data with a signal ID, they are useful in seeing whether or not the data has been tampered with, and can also be used as data one signs in order to prove authenticity of a document via other cryptographic means.
The ideal cryptographic hash function has four main properties:
* it is easy to compute the hash value for any given message.
* it is infeasible to find a message that has a given hash.
* it is infeasible to modify a message without changing its hash.
* it is infeasible to find two different messages with the same hash.
A cryptographic hash function must be able to withstand all known types of cryptanalytic attack. As a minimum, it must have the following properties:
* Preimage resistance: Given a hash h it should be hard to find any message m such that h = hash(m). This concept is related to that of one way function. Functions that lack this property are vulnerable to preimage attacks.
* Second preimage resistance : Given an input m1, it should be hard to find another input, m2 (not equal to m1) such that hash(m1) = hash(m2). This property is sometimes referred to as weak collision resistance. Functions that lack this property are vulnerable to second preimage attacks.
* Collision resistance : It should be hard to find two different messages m1 and m2 such that hash(m1) = hash(m2). Such a pair is called a (cryptographic) hash collision, and this property is sometimes referred to as strong collision resistance. It requires a hash value at least twice as long as what is required for preimage-resistance, otherwise collisions may be found by a birthday attack.